In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. An image reconstruction algorithm which may be utilized in reconstructing an image from data obtained in a helical scan is described in U.S. patent application Ser. No. 08/436,176, filed May 9, 1995, and assigned to the present assignee.
The projection data gathered with fan-beam helical scan can be denoted as P(.theta.,.gamma.,z) where .theta. is the angle of the central ray of the fan beam with respect to some reference (e.g., the y axis), .gamma. is the angle of a particular ray within the fan beam with respect to the central ray, and z is the axial gantry position at the time the measurement is made. For each location z.sub.0 at which actual projection data is not obtained, a commonly used and known helical reconstruction algorithm produces raw data for a slice at location z.sub.0 by using linear interpolation in the z direction. Specifically, to produce P(.theta.,.gamma.,z.sub.0), projection data at the same .theta. and .gamma. and as close as possible, but on opposite sides in z, to z.sub.0 are used. For example, if z.sub.1 and z.sub.2 are the values of z for which P(.theta.,.gamma.,z) are available, and for which z.sub.1 .ltoreq.z.sub.0 .ltoreq.z.sub.2, P(.theta.,.gamma.,z.sub.0) may be estimated from P(.theta.,.gamma.,z.sub.0 and P(.theta.,.gamma.,z.sub.2) by linear interpolation using the following: ##EQU1##
In a helical scan, since the same ray is measured twice in each 360.degree. rotation, i.e., P(.theta.,.gamma.,z)=P(.theta.+2.gamma.+180.degree.,-.gamma.,z), the z sampling is effectively doubled. This increased sampling enables reducing the total scan time.
It is desirable, of course, to reconstruct images from the data obtained in a four beam helical scan in a manner which provides a high quality image with a low level or number of artifacts. It also is desirable to reduce the total time required to reconstruct such an image. Further, since data may not necessarily be obtained for every axial location, it would also be desirable to provide an algorithm to estimate such projection data in a manner which enables generation of a high quality image.